Method of form removal from precision casting shells



' tion and claims.

United States Patent Ohio No Drawing. Filed Mar. 7, 1950, Ser. No.12,899 4 Claims. (Cl. 22-1fi5) This invention relates to the ceramicshell process of investment casting, and relates more specifically tothe use of plastic patterns and the process of removing such a patternfrom a shell coating formed thereon.

The object of this invention is to produce a ceramic casting shell froma plastic pattern.

Another object of this invention is to provide a process for removingthe plastic pattern without disrupting the thin ceramic shell.,

Other objects and a fuller understanding of the invention may be had byreferring to the following descrip- ,In the conventional investmentcasting lost wax process, a disposable pattern, which is a replica ofthe part to be cast including the necessary risers and gates, is coatedwith a refractory slurry which hardens and forms the mold face. The bulkof the mold is formed by a refractorycastable investment which is pouredor vibrated around the coated pattern in a suitable container known.generally as a flask, and which subsequently sets to a strong, hardmass.

The completed mold is then dried and heated to melt out the bulk of thepattern material. It is then heated still higher to burn off orvolatilize the last residue of the pattern material and to prepare themold to receive the molten metal.

The coating of the pattern is usually carried out by dipping it into asuspension of fine refractory powder in a suitable bonding liquid. Thebinder is one which is capable of hardening during drying at roomconditions. After dipping, the excess slurry is drained oif and thepattern is sprinkled or sanded with coarser refractory particles toplace the coating and to facilitate bonding between the coating and thebackup investment.

A growing trend within the investment casting industry is the use ofthin ceramic shell molds in place of the conventional bulky investmentmolds. The shell molds are usually prepared by repeating the dipping andsanding operation described above until a sufiicient coating thicknessis obtained to resist the stresses that occur in subsequent processing.The usual thickness is from /8,-

of an inch to /2 of 'an inch, although either thinner or heavier shellsmay be indicated for special situations.

Some of the advantages that are frequently obtained with shell moldsover conventional investment molds include:

( l) Lighter molds, easier to handle.

(2,) Increased permeability.

(3.) Increased thermal shock resistance.

(4) Easier knockout and cleanup after casting.

(5) No flasks required.

(6) Greater flexibility in processing.

A critical stage in the processing of such shells is the elimination ofthe pattern material. As the shell containing the pattern is heated, theusual pattern materials expand at a much higher rate than the shell, sothat the resultant expansion forces tend to crack the shell. This hasbeen overcome in the case of wax patterns by a procedure known as flashdewaxing. In this procedure the shell is placed directly into a furnaceat an elevated temperature, i.e. 1600-1800 F. Under these conditions theheat transfer through the shell is so rapid that a surface skin of waxmelts before the bulk of the wax can heat up enough to crack the shell.As the bulk of the wax does heat up, the molten surface material eitherflows out of the mold or soaks into the shell. This provides a space toaccommodate the bulk expansion of the wax, so that it will not crack theshell.

Attempts to use plastics in place of Wax patterns in shell molds havenot been successful due to the fact that plastics do not respond toflash dewaxing as waxes do. The term plastic is used herein as it isused in the trade to denote synthetic resins. Possibly the surface skindoes not soften as it should, or if it does, is not fluid enough to soakinto the mold, so that the shell is still subject to large pressures. Insome cases, strains in the plastic, which are set up during theinjection cycle, may be released during heating, causing a distortion ofthe plastic which may also crack the shell. Whether these explanationsare correct or not is not certain. But it is a fact that plasticpatterns will cause cracking of shells which would be quite suitable foruse with wax patterns. For example, a shell composition which can beused for wax patterns weighing over twelve pounds cannot be used with aplastic pattern weighing less than one-tenth of an ounce, vdue tocracking during burn out.

There are many instances whereit is desirable to use plastic patterns.Plastic patterns can be turned out on automatic injection equipment atmuch higher production rates than can be attained in conventional waxinjection operations. In addition, plastic patterns are not brittle, anddo not crack at'low temperatures or soften at high temperatures normallyencountered.

.The object of this invention is to provide a means of processingpatterns made up in whole or in part of plastic, in the same type ofshells that are now used for wax patterns. This is accomplished bysoaking the shells in a solvent which is capable of diffusing throughthe shell and softening the plastic inside, prior to the pattern.volatilization operation. The solvent soaking step is preferably carriedout before the dipping process is completed, since the required soakingtime is much shorter when the shell is thin. After the shell has beensoaked long enough to accomplish the desired purpose, the dipping iscompleted and the shell is subsequently handled by any procedure whichwould be suitable for wax patterns.

Further, it is possible to expel the softened plastic pattern by meansother than oven-heating. Dielectric heating is just one example.

This invention is not to be confused with prior known methods, whereinwax patterns were removed by solvents which fully dissolved the pattern.This invention is useful with patterns not fully soluble and further,the mere rendering of a pattern impotent with respect to its ability toresist an expansion force under later heat removal is far more efficientand less expansive.

It has been found that the diffusion of the solvent into the plasticcreates a pressure of its own. This pressure is small compared to thatgenerated during heating, but it is necessary that a minimum number ofdips be applied before the soaking operation can be carried outsuccessfully. This number varies, depending on the particularplastic-solvent-shell combination being used, and

suspension of zircon (zirconium silicate) and fused silica in a bondingliquid consisting mainly of a colloidal silica sol, with small amountsof an organic film former, a wetting agent, and a defoaming agent. Eachdip was sanded with a coarse fire-clay grog of -20+50 mesh.

Three dips were applied one hour apart. The shell was dried overnight,then soaked four hours in methyl ethyl ketone. It was removed from themethyl ethyl ketone, allowed to stand one hour, and then three more dipswere applied-one hour apart. After drying at room conditions, the shellwas dewaxed by placing it directly into a furnace at 1800 F. There wasno cracking or other defects. A similar shell, processed in the samemanner except that the solvent soaking was not used, cracked badlyduring dewaxing on the portion of the shell defining the plasticpattern. Furthermore, it was found that even twelve such coats did notbuild up a sufficient thickness to process successfully, without asolvent soaking.

A smaller plastic pattern weighing 2.5 grams was set up on a wax base.Six dips were applied as above, with a six hour soaking period inbenzene after the third dip. The shell was dewaxed successfully at 1800F. A similar shell with no solvent treatment cracked during dewaxing.Larger set ups having 25 such pieces have also been processedsuccessfully by the solvent soaking technique.

The soaking time required depends on such factors as size and shape ofthe plastic piece, number of pieces on the set up, type of plastic,solvent and-shell, number of dips before soaking and number of clips tobe applied after dipping. A peculiar effect that has been observed isthat when the soaking time is insufiicient, the shells normally will notcrack as untreated shells do, but the portions defining the plastic partof the pattern will blow apart. For example, test patterns were givensoaking periods of zero up to five hours after the third dip. Three moredips were applied after soaking. The shell with no soaking time crackedduring dewaxing in a very characteristic manner. Those soaked /2 tothree hours blew apart on the plastic portion. Those soaked four hoursor more were dewaxed successfully. The phenomenon has been noted onother types and sizes of set-ups as well.

Although the invention has been described in its preferred form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form has been made only by way of exampleand that numerous changes in the details of construction and thecombination and arrangement of parts may be resorted to withoutdeparting from the spirit and the scope of the invention as hereinafterclaimed.

I claim:

1. The process of producing a molding shell for the casting of shapestherein, comprising the steps of providing a synthetic resin, providingan agent capable of softening said synthetic resin after it is moldedand hardened, said agent being capable of producing a condition ofsoftness wherein the softened synthetic resin has a low degree ofexpansion force under heat of an intensity to volatize the syntheticresin, molding said synthetic resin into the form of a desired casting,providing a porous refractory shell coating over said synthetic resin,said shell having a strength greater than said low degree of softenedsynthetic resin expansion force, thereafter softening said syntheticresin by applying said softening agent through said porous shellcoating, and finally heating the softened form above the heatvolatilization temperature thereof until fully dispersed from the shellcoating.

2. The process of producing a molding shell for the casting of shapestherein, comprising the steps of, providing a synthetic resin and anagent capable of softening said synthetic resin after it is molded andhardened to a condition of softness wherein the softened synthetic resinhas a low degree of expansion force under heat of intensity to volatizethe synthetic resin, molding said synthetic resin into the form of adesired casting, providing a thin porous refractory shell coating oversaid synthetic resin having a strength greater than said low degree ofexpansion force, thereafter softening said synthetic resin by applyingsaid softening agent through said porous shell coating, thereafterapplying additional refractory shell coating over the previous coatingto increase the shell strength, and finally heating the softened formabove the heat volatilization temperature thereof until fully dispersedfrom the shell coating.

3. The process of producing a molding shell for the casting of shapestherein, comprising the steps of, providing a heat volatizable patternmaterial and an agent capable of softening a pattern made therefrom to acondition of softness wherein the softened pattern has a low degree ofexpansion force under heat of intensity to volatize the pattern, moldingsaid material into the form of a desired casting, providing a refractoryshell coating over said form having a strength greater than said lowdegree of expansion force, thereafter introducing said softening agentinto the shell and thus causing the softening agent to act upon thepattern within the shell, and finally heating the softened form abovethe heat volatilization temperature thereof until fully dispersed .fromthe shell coating.

4. The process of producing a molding shell for the casting of shapestherein, comprising the steps of, providing a polystyrene pattern,providing, a suspension of zirconium silicate and fused silica in abonding agent of colloidal silica sol as a shell forming dip, dippingsaid pattern in said dip material and thereafter stuccoirig the wet dipcoat with a coarse fire clay grog, allowing said stuccoed coat to dryand thereafter repeating the dip and said stucco step until at leastthree coats are built up, providing a bath of methyl ethyl ketone,submersing said coated pattern in said bath until said pattern issoftened, draining the excess methyl ethyl ketone and there afterapplying at least three further dip coatings, and finally placing saidcoated pattern into a furnace at llili00 F. until all the pattern isevaporated from the s ell.

Doolittle (The Technology of Solvents & Plasticizers) (copyright, 1954,by Union Carbide & Carbon Corp.) (TP 247.5, D6, C.2) (N.Y., John Wiley &Sons, Inc.)

(p. 452 relied upon).

ATES PATENT. OFFICE UNITED ST CERTIFICATE OF CORRECTION Patent N00 3,0I852B January 3O 1962 Robert A Horton hat error appears in the ab dLetters Paten certified t We numbered patt should read as d that the saiIt is hereby tion an ent requiring correc corrected below Column 3 line32 for "dipping" read soaking d this 26th day of June 1962,

Signed and scale SEAL) Attest:

DAVID L. LADD Commissioner of Patents ERNEST W SW IDER Attesting Officer

1. THE PROCESS OF PRODUCING A MOLDING SHELL FOR THE CASTING OF SHAPESTHEREIN, COMPRISING THE STEPS OF PROVIDING A SYNTHETIC RESIN, PROVIDINGAN AGENT CAPABLE OF SOFTENING SAID SYNTHETIC RESIN AFTER IT IS MOLDEDAND HARDENED, SAID AGENT BEING CAPABLE OF PRODUCING A CONDITION OFSOFTNESS WHEREIN THE SOFTENED SYNTHETIC RESIN HAS A LOW DEGREE OFEXPANSION FORCE UNDER HEAT OF AN INTENSITY TO VOLATIZE THE SYNTHETICRESIN, MOLDING SAID SYNTHETIC RESIN INTO THE FORM OF A DESIRED CASTING,PROVIDING A POROUS REFRACTORY SHELL COATING OVER SAID SYNTHETIC RESIN,SAID SHELL HAVING A STRENGTH GREATER THAN SAID LOW DEGREE OF SOFTENEDSYNTHETIC RESIN EXPANSION FORCE THEREAFTER SOFTENING SAID SYNTHETICRESIN BY APPLYING SAID SOFTENING AGENT THROUGH SAID POROUS SHELLCOATING, AND FINALLY HEATING THE SOFTENED FORM ABOVE THE HEATVOLATILIZATION TEMPERATURE THEREOF UNTIL FULLY DISPERSED FROM THE SHELLCOATING.